The present invention relates to a method and a system for inspecting wafers for electronics, optics or optoelectronics.
During the manufacture and use of wafers for electronics, optics or optoelectronics, it is usual to carry out an inspection of the surface of each wafer so as to detect any defects.
On account of the very small size of the defects to be detected, a visual inspection by an operator is not sufficient.
Furthermore, the inspection is generally intended not only to discover the presence or absence of defects, but also to provide qualitative and/or quantitative information on said defects, such as their location, their size and/or their nature, for example.
Inspection systems have thus been developed with a view to detecting increasingly small defects and to provide all required information on the nature, the size, the location, etc. of said defects.
These systems must also allow a duration of inspection of each wafer that is sufficiently short so as not to adversely affect production speeds.
Document WO 2009/112704 describes a system for inspecting semi-conductor wafers implementing Laser Doppler Velocimetry (LDV). As shown in FIG. 1, this system comprises a light source 20 and an interferometric device 30 coupled with the light source arranged facing the surface S of the wafer 2 for inspection, which is actuated by a movement. Said interferometric device comprises a light guide the input of which is coupled with the light source and comprising two branches for dividing the beam originating from the light source into two incident beams. At the output of the light guide, the two branches are oriented in relation to one another so as to form, at the intersection between the two beams, a measurement volume comprising a plurality of parallel fringes. The system also comprises an optical fibre 40 arranged between the surface of the wafer and a detection module 50, so as to guide the light backscattered by the surface of the wafer towards the detection module.
Document WO 02/39099 describes another system for inspecting semi-conductor wafers relying on Laser Doppler Velocimetry.
The presence of a defect on the surface of the wafer is indicated, when this defect crosses the interference fringes, by the scattering of a Doppler burst measured by the detection module. A Doppler burst is a signal that has a double frequency component: a low-frequency component, forming the envelope of the signal, corresponding to the mean light intensity scattered by the defect, and a high-frequency component, corresponding to the Doppler frequency containing the information on the velocity of the defect. The Doppler frequency fD is linked to the velocity v of movement of the defect in the direction perpendicular to the interference fringes and to the distance Δ between the interference fringes (or inter-fringe distances) by the relationship v=f*Δ.
FIG. 2 shows a Doppler burst due to a defect passing through the interference zone, expressed in the form of an electrical voltage (in Volts) at the output of the detection module as a function of time.
On the basis of such a Doppler burst, it is possible to determine the size of the defects detected on the surface of the wafer.
In this respect, reference may be made to the publication by W. M. Farmer entitled “Measurement of Particle Size, Number Density, and Velocity Using a Laser Interferometer”, which presents a model of the visibility of a particle as a function of the particle size.
Thus, for a pattern of given interference fringes, the relationship between the size of a defect compared to a sphere, which is defined as the diameter of the sphere, and the visibility determined according to the above formula, is given by a curve of the type shown in FIG. 3.
It is noted that, for a visibility greater than 0.15, the curve of FIG. 3 provides a unique defect size corresponding to a given visibility value.
However, for a visibility less than 0.15, the curve shows “bounces”, indicating the fact that a single visibility value can correspond to several defect sizes. Thus, in the example in FIG. 3, a visibility of 0.1 corresponds to three radii of a sphere: 0.83 μm, 1.12 μm and 1.45 μm.
In such a case, the problem then arises of determining, among these different possible sizes, the actual size of the defect present on the wafer.
In particular, this technique does not allow measurement of the size of defects of very different sizes. In fact, as shown in FIG. 3, it is not possible to determine the size of defects having a size larger than 0.9 μm (corresponding to a visibility less than 0.15).
Now, the size of the defects capable of being detected on a wafer extends over a wide range of dimensions, typically from a few tens of nanometers to a few hundred micrometers.
Another drawback of the technique based on the curve in FIG. 3 is that, for some defect sizes (for example a radius of 0.95 μm), the visibility is zero, i.e. no Doppler burst is produced. Consequently, a defect of this size cannot be detected.